Re: [patch 4/5] sched: Delay task stack freeing on RT

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Sep 29, 2021 at 4:54 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Sep 28, 2021 at 02:24:30PM +0200, Thomas Gleixner wrote:
>
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -172,6 +172,11 @@ static void delayed_put_task_struct(stru
> >       kprobe_flush_task(tsk);
> >       perf_event_delayed_put(tsk);
> >       trace_sched_process_free(tsk);
> > +
> > +     /* RT enabled kernels delay freeing the VMAP'ed task stack */
> > +     if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +             put_task_stack(tsk);
> > +
> >       put_task_struct(tsk);
> >  }
>
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4846,8 +4846,12 @@ static struct rq *finish_task_switch(str
> >               if (prev->sched_class->task_dead)
> >                       prev->sched_class->task_dead(prev);
> >
> > -             /* Task is done with its stack. */
> > -             put_task_stack(prev);
> > +             /*
> > +              * Release VMAP'ed task stack immediate for reuse. On RT
> > +              * enabled kernels this is delayed for latency reasons.
> > +              */
> > +             if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> > +                     put_task_stack(prev);
> >
> >               put_task_struct_rcu_user(prev);
> >       }
>
>
> Having this logic split across two files seems unfortunate and prone to
> 'accidents'. Is there a real down-side to unconditionally doing it in
> delayed_put_task_struct() ?
>
> /me goes out for lunch... meanwhile tglx points at: 68f24b08ee89.
>
> Bah.. Andy?

Could we make whatever we do here unconditional?  And what actually
causes the latency?  If it's vfree, shouldn't the existing use of
vfree_atomic() in free_thread_stack() handle it?  Or is it the
accounting?


-- 
Andy Lutomirski
AMA Capital Management, LLC